Construction of an optical fiber communications system requires a single-mode waveguide device having various functions. The ion exchange method is one of the processes that can provide such a device at low cost. The present inventors previously newly proposed a two-stage purely-thermal ion exchange method, and demonstrated its usefulness. The process of this method is described in Electronics Letters, Vol.24, p.1258 (1988), and preferred preparation conditions are disclosed in JP-A-63-206709. (The term "JP-A" as used herein means an "unexamined published Japanese patent application") By using this method, a single-mode waveguide which shows a coupling loss of 0.25 to 0.35 dB when coupled to a standard single-mode optical fiber with a mode field diameter of about 9 .mu.m and also has a transmission loss of about 0.1 to 0.2 dB/cm can be produced relatively easily.
However, the conventional process described above has been insufficient in that the coupling loss of the waveguide obtained is somewhat high. For this reason, it has been a little difficult to produce devices having extremely low losses with an excess loss of 1 dB or less. Although this problem can be eliminated by the conventionally known electric field-applying ion exchange method, particularly by the two-stage electric field ion exchange method in which an electric field is applied at least during the second-stage ion exchange. However, this troublesome production process must be employed to attain sufficiently low losses, so that low-cost mass production has been impossible.